Hydrocarbon isomerization and separation process

ABSTRACT

A process for isomerizing and separating a low octane C5-C6 hydrocarbon charge stock to provide a branched hydrocarbons product in which the charge stock is contacted with an isomerization catalyst in an isomerization zone, the effluent from the isomerization zone is separated, in a sorption zone employing a molecular sieve sorbent, into a branched hydrocarbons stream and a normal hydrocarbons stream; the branched hydrocarbons stream is fractionated, and a methylpentanes-free stream of branched hydrocarbons is recovered from the fractionation step as the product of the process.

United States Patent [1 1 Asselin HYDROCARBON ISOMERIZATION ANDSEPARATION PROCESS [75] Inventor: George F. Asselin, Mt. Prospect, I11.

[73] Assignee: Universal Oil Products Company, Des Plaines, 111.

[22} Filed: Oct. 13, 1971 [21] Appl. No.: 188,792

Make up Hydrogen Pen ans /Hexane Fee a lsomen'za t/on Reactor HighPressure Separa/ar [451 Aug. 28, 1973 Primary Examiner-Delbert E. GantzAssistant ExaminerS. Berger Att0rney.1ames R. Hoats'omJr. et a1.

' [57 ABSTRACT A process for isomerizing and separating a low octane C-C hydrocarbon charge stock'to provide a branched hydrocarbons productin which the charge stock is contacted with an isomerization catalyst inan isomerization zone, the effluent from the isomerization zone isseparated, in a sorption zone employing a molecular sieve sorbent, intoa branched hydrocarbons stream and a normal hydrocarbons stream; thebranched hydrocarbons stream is fractionated, and a methylpentanes-freestream of branched hydrocarbons is recovered from the fractionation stepas the product of the process.

5 Claims, 1 Drawing Figtire L ighr Gases /sapenfane D/mef/zy/bu/aneProducf Ma/ecu/ar S/e v19 Sepamr/on Zane HYDROCARBON ISOMERIZATION ANDSEPARATION PROCESS BACKGROUND This invention relates to a process forisomerizing and separating a lower octane C C hydrocarbon charge stockcontaining normal paraffinic hydrocarbons to provide higher octaneproducts. In one aspect, this invention relates to a hydrocarbonisomerization process utilizing fractionation and a solid sorbent toseparate branched chain C and C hydrocarbons from normal C and C,hydrocarbons.

More specifically, this invention relates to a process for isomerizingand separating an isomerizable hydrocarbon charge stock containinghydrocarbons having from about five to about six carbon atoms permolecule to provide a branched hydrocarbons product, which processcomprises: contacting said charge stock with an isomerization catalystin an isomerization zone at isomerization conditions; separating theeffluent from said isomerization zone in a sorption zone, employing asolid sorbent, to provide a normal hydrocarbons stream and a branchedhydrocarbons stream; and, fractionating said branched hydrocarbonsstream to provide a methylpentanes stream and a products stream andwithdrawing said products stream as said branched chain hydrocarbonsproduct.

Hydrocarbons comprising primarily pentanes and hexanes, such as thoseavailable in petroleum refineries, for example, as straight-runfractions of crude oil or as raffinate from reforming processes,represent a substantial potential source of high octane motor fuelcomponents. However, these C -C fractions normally have an unleadedresearch octane rating of. only about 65-70, and the octane rating ofsuch fractions is raised to only about 85-90 by the addition of 3 ml. ofan alkyl lead compound per gallon of hydrocarbon. Present premium fuelsfor automobile engines must have an octane rating of about 100. Onemethod of upgrading these C -C fractions, to improve their utility asmotor fuel components, is by eliminating low octane normal paraffinsfrom them by isomerizing the normal paraffms to higher octane branchedisomers. It is well known that the more highly branched hydrocarbonssuch as isopentane and the dimethylbutanes possess a higher unleadedoctane than do nonnal pentane and normal hexane. For example, isopentaneand 2,2-dimethylbutane each have an unleaded research octane of about93, while n-pentane and n-hexane have unleaded octane ratings of about62 and 30 respectively. By eliminating at least a part of the n-pentaneand n-hexane from a typical C -C refinery fraction, the octane rating ofthe fraction can, therefore, be substantially improved. With the adventof a strong demand for unleaded motor fuel for automobiles, there is aneven more pressing need for motor fuel components which have a highoctane rating without the addition of alkyl leads. It is essential toupgrade the available C -C, fractions if they are to be useful inunleaded motor fuels. The process herein disclosed provides a novel andeconomically superior method for producing high octane products fromthese low octane petroleum fractions.

lsomerization of hydrocarbons is well known in the petroleum refiningart. Generally, hydrocarbon stocks have been isomerized and fractionatedto recover valuable components. The combination of isomerization withfractionation and molecular sieve separation disclosed in the process ofthis invention makes possible a sharper separation, smallerisomerization reactor requirement and higher quality product than isgenerally available from current commercial processes.

SUMMARY 7 It is an object of this invention to provide a process forisomerizing and separating a lower octane pentanelhexane hydrocarbonfraction to produce a higher octane hydrocarbon product. Another objectof this invention is to provide a hydrocarbon isomerization processutilizing a solid molecular sieve sorbent to separate normalhydrocarbons from branched and cyclic hydrocarbons to produce a higheroctane, more highly branched product from a lower octane, less branchedhydrocarbon fraction.

Therefore, in an embodiment, this invention relates to a process forisomerizing and separating an isomerizable hydrocarbon charge stockcontaining hydrocarbons having from about five to about six carbon atomsper molecule to provide a branched hydrocarbons product, which processcomprises: (a) containing said charge stock with an isomerizationcatalyst in an isomerization zone at isomerization conditions; (b)separating the effluent from said isomerization zone in a sorption zone,employing a solid sorbent, to provide normal hydrocarbons stream and abranched hydrocarbons stream; and, (c) fractionating said branchedhydrocarbons stream to provide a methylpentanes stream and a productsstream and withdrawing said products stream as said branched chainhydrocarbons product.

Further objects and embodiments of the process of this invention will bemore readily apparent from the following description of the accompanyingdrawing and detailed description of the invention.

DESCRIPTION OF THE DRAWING An understanding of the process of thisinvention may be aided by referring to the accompanying drawing whichrepresents a schematic diagram of an embodiment of the process of thisinvention.

Referring to the drawing, hydrocarbon charge stock, comprising primarilypentanes and hexanes, is introduced through conduit 1 and passed throughconduit 2 to heat exchanger 3 wherein the charge stock is heated inindirect heat exchange with effluent from isomerization zone 7. Hydrogenis commingled with the charge stock in conduit 2. The partially heatedcharge stock is withdrawn from exchanger 3 and passed through conduit 4to heater 5 wherein the charge stock is further heated to' the desiredtemperature for isomerization conditions. The heated charge stock ispassed from heater 5 via conduit 6 to isomerization reactor 7.Isomerization reactor 7 contains a fixed bed of isomerization catalyst.The hydrocarbons charged to reactor 7 are passed over the fixed bed ofcatalyst, maintained at a temperature and pressure sufiicient that atleast a part of the charged hydrocarbons are isomerized, and withdrawnfrom reactor 7 through conduit 8. The isomerization reactor effluent ispassed from conduit 8 into heat exchanger 3 in indirect heat exchangewith fresh feed stock as described above. Cooled reactor effluent iswithdrawn from heat exchanger 3 and passed through conduit 9 to highpressure separator 10, wherein the effluent forms a liquid phase and ahydrogen-rich gaseous phase. The hydrogen-rich gaseous phase iswithdrawn from separator 10 through conduit 11. Make up hydrogen ischarged to conduit 11 by way of conduit 12 as needed, and the combinedhydrogen stream continues through conduit 11 to conduit 2 where thehydrogen is commingled with fresh charge stock as described above. Theliquid phase in high pressure separator is withdrawn through conduit 13and passed to stabilizer 14. In stabilizer 14, any light gases, such asbutanes, propane, ethane and methane, as well as any other normallygaseous materials such as hydrogen halides, are separated from C,,/Chydrocarbons, passed overhead in conduit 15 and withdrawn from theprocess. The C .,/C hydrocarbons in stabilizer 14 are withdrawn andpassed through conduit 16 to molecular sieve separation zone 17. Inseparation zone 17, normal C and C hydrocarbons are separated frombranched chain and cyclic C and C hydrocarbons. The normal hydrocarbonsare withdrawn via conduit 18 and passed therethrough to conduit 2,wherein they are commingled with fresh charge stock from conduit 1 andhydrogen from conduit 11. The branched and cyclic hydrocarbons inmolecular sieve separation zone 17 are withdrawn through conduit 19 andpassed to deisohexanizer 20, which comprises a fractionation vessel. Indeisohexanizer 20, isopentane, dimethylbutanes and some methylpentanesare withdrawn overhead in the vapor phase and recovered through conduit21 as the product of the process. A bottoms stream comprising primarilymethylcyclopentane, and cyclohexane is recovered from deisohexanizer andwithdrawn from the process through conduit 23. A side cut stream ofDETAILED DESCRIPTION OF INVENTION The hydrocarbon charge stocks whichare suitable for use in the process of this invention include straightand branched chain saturated C and C hydrocarbons and mixtures thereof.Typical of the suitable charge stocks is a C /C fraction derived fromthe fractional distillation of petroleum crude oil. Such a fraction maycontain lighter and heavier components such as butanes and heptanes andmay contain cyclic and aromatic compounds such as cyclopentane,cyclohexane, benzene, etc. Typically, such straight-run fractions arecontaminated by sulfur compounds and water, and it is desirable toeliminate such contaminants before a charge stock is utilized in thisprocess. Generally, the charge stock should comprise less than about 10wt. percent of C-, and heavier hydrocarbons, although this requirementis not essential to the operation of the process. It is also desirableto limit the aromatics content of the charge stock to less than about 10percent of the feed, by weight, primarily because the aromatics havehigher octane rating than the products formed from them in theisomerization reaction. Suitable charge stocks may contain unsaturatedhydrocarbons, such as pentenes, hexenes, pentadienes, etc. When thecharge stock is obtained from the raffinate of a hydrocarbon reformingprocess, it will usually contain several wt. percent of suchunsaturates. Such a feed stock is suitable for use in the presentprocess. It will be obvious to one skilled in the art that the presentprocess is basically applicable to C JC charge stocks containing agreater-than-equilibrium fraction of normal pentane and normal hexane,since the octane rating improvement resulting from processing stockshaving a high branched chain, cyclic and aromatic hydrocarbons contentwould be negligible. A typical straight run C,,/C feed stock, suitablefor use as a feed in the present process, could comprise, by weight, 20percent isopentane, 30 percent n-pentane, 1 percent cyclopentane, 2percent dimethylbutanes, 19 percent methylpentanes, 19 percent n-hexane,5 percent methylcyclopentane, 1 percent benzene and 3 percent heavierhydrocarbons.

Isomerization catalysts suitable for use in the process of thisinvention include all catalysts capable of isomerizing normal paraffinichydrocarbons to produce branched chain parafiinic hydrocarbons.Friedel-Crafts metal halides, e.g., aluminum chloride, are well known asisomerization catalysts. These catalysts may be used as such or combinedwith other materials such as hy drogen halides. Typical catalysts ofthis type, suitable for use in the present process, are aluminumchloride with hydrogen chloride added as a promoter and antimonychloride having aluminum chloride dissolved therein. Friedel-Craftsmetal halides may be utilized suitably in combination with substantiallyinert supporting materials. Another suitable type of isomerizationcatalyst which may be used in this process is a composite of a metalfrom Group VIII of the Periodic Table with a solid support. The platinumgroup metals, and platinum in particular, are preferred for use ascomponents of such a catalyst in this process. Solid supports which aresuitable for a catalyst of this type include silica, alumina, magnesia,zirconia, chromia, titania, bauxite, kaolin, Kieselguhr, vanadium oxide,boron oxide, etc., and combinations thereof. Catalysts comprising aGroup VIII metal component and a solid support may be utilized withpromoting components such as hydrogen halides, organic halides, etc. Forexample, the composite may be treated, before or during use, withhydrogen halide or with an organic halide such as carbon tetrachloride,chloroform, etc. A hydrogen halide or organic halide may be admixed withthe hydrocarbons to be isomerized in a continuous operation whereby adefinite concentration of halide is provided in the feed to theisomerization zone. A similar type of catalyst suitable for use in theprocess of this invention comprises a Group VIII metal component and oneor more combined halogen components on a solid support. For example, acatalyst comprising, by weight, about 0.01 to about 2.0 percentplatinum, about 0.01 to about 8 percent chlorine, about 0.01 to about 8percent fluorine on a silica-alumina support is suitable for use in thisprocess. One isomerization catalyst preferred for use in the presentprocess is described in U. S. Pat. Ser. No. 2,999,074. This preferredcatalyst comprises a calcined and reduced composite of a refractoryoxide containing chemically combined hydroxyl groups and from about 0.01percent to about 2 percent by weight of a platinum group metal, saidcalcined composite being impregnated with from about 10 percent to aboutpercent by weight of an anhydrous Frie del-Crafts metal halide in whichthe halogen is selected from chlorine and bromine and said hydroxylgroups having been reacted with the metal halide in a reaction resultingin the elimination from the composite of at least 0.5 mole but not morethan 2.0 moles of hydrogen Various other suitable catalysts have beendisclosed in prior art. For example, catalytic composites containingmore than one metallic component, e.g., a platinum group metal and ametal from Group VIB or Group IB of the Periodic Table have been foundto'be useful as isomerization catalysts. Certain forms of crystallinealuminosilicates have also been found to act as isomerization catalysts,and may suitably be used as such in the process of this invention. Suchcatalytic composits commonly require isomerization conditions verysimilar to those discussed below in connection with the use of thepreferred catalysts, so that the isomerization conditions discussed aregenerally applicable to them.

Another catalyst preferred for use in the process of this inventioncomprises a halogen component on a solid support comprising an aluminamatrix having suspended therein a finely divided crystallinealuminosilicate and a Group VIII or Group VIB metallic component. Such acatalyst may be prepared by distributing finely divided crystallinealuminosilicate particles throughout an aluminum hydroxyl halide sol, toform a mixture, gelling the resulting mixture to form substantiallyspherical hydrogel particles, aging, washing and calcining theparticles, and adding platinum thereto. The crystalline aluminosilicate,preferably mordenite, should comprise from about 2 percent to aboutpercent by weight, of the support, and the platinum component shouldcomprise from about 0.05 percent to about 2.0 percent of the finalweight of the catalyst.

In using the preferred isomerization catalysts, it is desirable toeliminate sulfur and water from the feed stock before it is contactedwith the catalyst. Typically, C,,-C, fractions available forisomerization have been desulfurized by prior processing, but, ifnecessary, sulfur may be removed by contacting the feed stock withhydrogen at elevated temperatures and pressures, whereby the sulfur isconverted to hydrogen sulfide and is easily separated from the heavierhydrocarbons. Various methods for eliminating water from the feed to theprocess are known including pre-fractionation and molecular sievedrying. The latter method is preferred. Dual function isomerizationcatalysts such as the preferred catalysts require that hydrogen gas bepresent in the isomerization zone. It is necessary to eliminate waterfrom fresh hydrogen which will enter the isomerization zone. It ispreferred that the fresh hydrogen be dried using molecular sieves.

The isomerization step in the process of this invention may be performedin a batch reaction scheme or a continuous reaction scheme. Theparticular manner in which the isomerization step is performed willdepend, in part, on the choice of isomerization catalyst. Generally, acontinuous operation is preferred. When a batch operation is utilized, aquantity of the charge stock is placed in an appropriate vessel andcontacted therein with an isomerization catalyst. The stock ismaintained at a particular temperature and pressure for a predeterminedlength of time and then separated from the catalyst and recovered. In acontinuous scheme, the catalyst may be employed as a fixed bed in asuitable reaction vessel and the hydrocarbon stock passed across the bedcontinuously. In another suitable method, the catalyst may be employedin a moving bed scheme, or a fluidized bed operation in which catalystand hydrocarbons are continuously contacted, either cocurrently orcountercurrently, and separated. It is preferred that a fixed bed ofcatalyst be maintained iii the reaction zone.

Isomerization conditions include a temperature in the range from about0F. to about 1,200F. and a pressure in the range from 1 atmosphere toabout 200 atmospheres or more. Preferred isomerization conditionsinclude a temperature of from about 200F. to about 800F. and a pressureof from about 10 atmospheres to about atmospheres. It is contemplatedthat gases such as helium, nitrogen, argon, etc.,may be charged to theisomerization zone if desired. In an embodiment of the present processusing one of the preferred catalysts, a gas comprising hydrogen ispassed to the iSOrfi-J erization zone and commingled with thehydrocarbon to be isomerized. Hydrogen should be charged at a rate inthe range from about 0.1 to about 10 moles per mole of chargedhydrocarbons. Hydrocarbons, either in the gaseous or the liquid phase,are passed through the isomerization zone at a liquid hourly spacevelocity (volume of hydrocarbons charged per hour divided by the volumeof catalyst in the isomerization zone) of from about 0.1 to about 20 ormore.

In using the preferred isomerization" catalysts, itis: preferred that anorganic halide-be addedtothe? feed to the isomerization zone.This-procedurere'sults in the production of some hydrogen halid'einthe'isoni'eriza tion zone, and beneficially affects the activity of thecatalyst. In embodiments of the presentp'roc'ess utilizing hydrogen,organic halide, etc., in the-isomerization" step, it is necessary toprovide equipmentforseparating hydrogen and other light gases from theheavier hydro;-

carbons. Various methods for making such aseparatio'ii are well known.One suitable metIiOdempIQystWo's'epQ aration zones, the first zoneoperatingathigh' pressure and the second .at a lowerpr'essure. In" thefirst,- highpressure separation zonea gaseous stie'an'i corrfpi'isin'gprimarily hydrogenis separated. The-hydrogen recov-' ered from the highpressure separation mneisrecirculated to the isomerization zone. In thesecond, lower" pressure separationzone, any C, and lighter'h'y'dro'carbons, hydrogen halides, etc., are separated from" the heavierhydrocarbons comprising primarily C and C saturates.

The isomerization zone effluent C, and heavier hydrocarbons are chargedto a sorption zone, wherein normal hydrocarbons are separated frombranched chain and cyclic hydrocarbons. The separation of norcrystalline aluminosilicate is a Type A zeolite manufac tured bythe-Linde'Di'vision of Union CarbidCoifii,

particularly the-5A zeolites of this'typei' Zeolites 'rnay becharacterized ashaving a porous structure,- with the pores beinginterconnected by smaller diameter pore? openings. When the poreopenings are ab'out 5-Ainidi ameter, normal hydrocarbons canenter"the'-pores,'but

cyclic and branched chain hydrocarbon's'cannot enter because of theirlarger molecular diameters. When-a;- mixture of normal, branched andcyclic hydrocarbons 11 is contacted with a zeolite of this type, thezoliteacts as a molecular sieve admitting normal hydrocarbons to thepores but excluding branched and cyclichydrocar bons. The branched andcyclic hydrocarbons are then withdrawn from contact with the zeoliterelatively free of normal hydrocarbons, and the relatively branchedchain-free normal hydrocarbons are subsequently desorbed from thezeolite.

The zeolite crystal structure consists of threedimensional networks ofthe fundamental units, which are silicon-centers SiO, andaluminum-centered A10 tetrahedra. These units are interconnected bysharing apical oxygen atoms. To preserve electrochemical neutrality, theA10 tetrahedra are associated with a cation such as an alkaline earth oralkali metal ion. Both natural and synthetic zeolites may be utilized asa sorbent in the process of this invention, including those in which A10tethedra are bonded with alkali and alkaline earth metal cations, aswell as those where the cations have been replaced by ion exchange ortreatment with acids or bases. The zeolite may be composited withbinding materials such as refractory inorganic oxides, preferably thoseinert with respect to catalytic and molecular sieve properties. Thesorbent may be employed in a batch, moving bed, fixed bed or other typeof operation. For example, one suitable type'of separation technique,employing a simulated moving bed operation, is described in U. S. Pat.No. 2,985,589.

In general, the use of a desorbent is required to recover the adsorbednormal hydrocarbons from a molecular sieve sorbent. Desorbents preferredfor use in the process of this invention include those which can beeasily separated from normal hexane. in operations making use of adesorbent, the preferentially adsorbed feed component, in this casenormal pentanes and hexanes, and the desorbent are removed from thesolid sorbent in admixture. Easy separation of the normal hydrocarbonsfrom the desorbent is, therefore, essential to the economy of theoperation. One preferred type of desorbent is a normal hydrocarbonhaving either a higher or lower boiling point than normal hexane ornormal pentane, respectively, such as n-octane or nbutane. Anotherpreferred type of desorbent is a mixture of a normal hydrocarbon and abranched hydrocarbon both having lower or higher boiling points thann-pentane or n-hexane such as a mixture of n-butane and isobutane,- or amixture of n-octane and isooctanes. Another method of desorbing thenormal hydrocarbons from the solid sorbent is by adsorbing the normalhydrocarbons at a higher pressure, and then desorbing them utilizing alower pressure to produce a vacuum effect. The normal hydrocarbons mayalso be desorbed by heating the sorbent, vaporizing the adsorbed normalhydrocarbons and withdrawing them. A desorbent may also be employed inoperations utilizing vacuum or heating desorption. For example, a gassuch as hydrogen, nitrogen, argon, etc., may be utilized as a desorbent.The desorbent and normal hydrocarbons are subsequently separated, forexample, by fractionation when a hydrocarbon desorbent is utilized, orby flash separation when a gas is used. A stream of relatively purenormal hydrocarbons is thereby produced and the desorbent recovered maybe recycled for further use.

In the present process, a raffinate of non-adsorbed hydrocarbons,comprising branched and cyclic C, and C, saturates is withdrawn fromcontact with the sorbent and passed out of the sorption zone, leavingthe adsorbed normal pentane and normal hexane. The adsorbed hydrocarbonsare subsequently desorbed, preferably by employing a desorbent. Thenormal pentane and normal hexane are then separated from the'desorbent,if any, and withdrawn from the sorption zone. The stream of normalhydrocarbons so formed is recycled to the isomerization step.

The stream of branched and cyclic hydrocarbons recovered as the sorptionzone raffinate is further separated by fractionation. It is desirable torecover a branched product comprising isopentane and the maximumfraction possible to dimethylbutanes from this fractionation step, whileretaining a recycle stream of as large a fraction of methylpentanes aspossible. The

very small difference in boiling point between 2,3- t.

dimethylbutane and 2-methylpentane makes a complete separation of theseisomers by fractionation virtually impossible. Depending upon theparticular embodiment of this invention, the product of the process maycomprise substantially methylpentane-free isopentane and2,2-dimethylbutane, or the product may comprise isopentane, 2,2- and2,3-dimethylbutane with a fraction of methylpentane. By variations infractionation conditions well known to those skilled in the art, theamount of methylpentanes taken overhead with the desired products may bevaried over a wide range, and the product of the process may be variedin octane rating accordingly. For example, if fractionation conditionsare imposed which provide an overhead product consisting essentially ofisopentane and 2,2- dimethylbutane, the product will possess an unleadedresearch octane of about 93. If much of the high octane2,3-dimethylbutane is also recovered overhead, it is also necessary totake overhead some methylpentane. In this embodiment, a product alsohaving an unleaded research octane of about 93 may be recovered, and asmaller and less costly recycle stream of methylpentanes will result.When fractionation conditions are adjusted to take overhead more thanabout 10 volume percent of the methylpentanes in the fractionator feed,the octane rating of the product will be correspondingly less, while therecycle stream will be smaller. Regardless of the desired octane of theproduct, it is essential to the concept of the present process that atleast a portion of the methylpentanes in the feed to the fractionationstep be separated from the product and, preferably, recycled to theisomerization step.

In general, the feed to the fractionation step will contain at leastsome methylcyclopentane and cyclohexane. These hydrocarbons willinvariably be retained in the recycle, bottoms portion of thefractionator. Although it may be possible to isomerize these cycliccompounds to the desired branched paraffinic isomers by recycling themto the isomerization step, it may be more economical to separate themfrom the recycle stream of methylpentanes and withdraw them from theprocess. One preferred method for accomplishing such a separation is towithdraw the recycle methylpentanes stream as a side cut from the lowersection of a frac tionation vessel and to withdraw the cyclics in thebottoms from the fractionator, while the product of the process isrecovered overhead. Thus the desired separation can be produced bysimply enlarging the fractionator necessary to the process.

I claim as my invention:

1. A process for the conversion of a C C, hydrocarbon fraction into morevaluable products which comprises isomerizing said fraction, separatingfrom the resultant effluent a C C. isomerate, contacting the latter witha solid sorbent to separate the isomerate into a C -C1 normalhydrocarbons stream and a C -C branched hydrocarbons stream,fractionating the last-mentioned stream to separate the same into anisopentane-dimethylbutane overhead product, amethylcyclopentane-cyclohexane bottoms product and a methylpentane sidecut, recycling said C -C nonnal hydrocarbons stream and said side cut tothe isomerizing step, and separately recovering said overhead productand said bottoms product.

2. The process of claim 1 further characterized in that said fraction isisomerized in contact with a catalyst comprising a refractory inorganicoxide component, a combined halogen component and platinum group metalcomponent.

3. The process of claim 1 further characterized in that said solidsorbent is a zeolitic molecular sieve-type sorbent.

4. The process of claim 3 further characterized in that said sorbentselectively adsorbs normal hydrocarbons and rejects branched chain andcyclic hydrocarbons.

5. A process for isomerizing and separating a lower octane isomerizablehydrocarbon charge stock containing hydrocarbons having from about fiveto six carbon atoms per molecule to produce a higher octane branchedhydrocarbons product, which process comprises the steps of:

a. contacting said charge stock with hydrogen and with a compositecomprising a refractory inorganic oxide component, a platinum groupmetal component and a combined halogen component, in an isomerizationzone at isomerization condition;

b. separating the effluent from said isomerization zone into ahydrogen-rich gaseous phaseand a liquid isomerate phase;

0. separating said isomerate phase in a sorption zone, utilizing a solidzeolite sorbent, to provide a normal hydrocarbons stream and a branchedhydrocarbons stream and introducing said normal hydrocarbons stream intosaid isomerization zone;

d. fractionating said branched hydrocarbons stream to provide a productstream comprising isopentane and dimethylbutanes, a recycle streamcomprising methylpentane, and a bottoms stream comprisingmethylcyclopentane and cyclohexane, and recovering said products streamas said branched hydrocarbons product; and,

(e) withdrawing said bottoms stream from the process and introducing atleast a portion of said recycle stream into-said isomerization zone.

2. The process of claim 1 further characterized in that said fraction isisomerized in contact with a catalyst comprising a refractory inorganicoxide component, a combined halogen component and platinum group metalcomponent.
 3. The process of claim 1 further characterized in that saidsolid sorbent is a zeolitic molecular sieve-type sorbent.
 4. The processof claim 3 further characterized in that said sorbent selectivelyadsorbs normal hydrocarbons and rejects branched chain and cyclichydrocarbons.
 5. A process for isomerizing and separating a lower octaneisomerizable hydrocarbon charge stock containing hydrocarbons havingfrom about five to six carbon atoms per molecule to produce a higheroctane branched hydrocarbons product, which process comprises the stepsof: a. contacting said charge stock with hydrogen and with a compositecomprising a refractory inorganic oxide component, a platinum groupmetal component and a combined halogen component, in an isomerizationzone at isomerization condition; b. separating the effluent from saidisomerization zone into a hydrogen-rich gaseous phase and a liquidisomerate phase; c. separating said isomerate phase in a sorption zone,utilizing a solid zeolite sorbent, to provide a normal hydrocarbonsstream and a branched hydrocarbons stream and introducing said normalhydrocarbons stream into said isomerization zone; d. fractionating saidbranched hydrocarbons stream to provide a product stream comprisingisopentane and dimethylbutanes, a recycle stream comprisingmethylpentane, and a bottoms stream comprising methylcyclopentane andcyclohexane, and recovering said products stream as said branchedhydrocarbons product; and, (e) withdrawing said bottoms stream from theprocess and introducing at least a portion of said recycle stream intosaid isomerization zone.